At present, low-quality coal, typically lignitous coal and low-rank coal, is abundant in reserve but is severely limited in application owing to its high moisture content and low heat value, etc., and has low efficiency and severe pollution during utilization, thus is in contradiction to the increasingly strict energy conservation and environment protection requirements in China. There is an urgent need for developing more efficient and reliable low-quality coal upgrading technology and equipment. Presently, air dense-medium fluidized beds and air dense-medium fluidized beds with an external force field can accomplish high efficient separation of low-moisture coal, but the dense medium particles may cohere and agglomerate under the impact of more external moisture and thereby may severely degrade fluidization performance of the fluidized bed owing to the fine granularity of the dense medium particles. Hence, these fluidized beds cannot carry out separation of low-quality coal that contains high moisture directly. Furthermore, even though a pre-drying procedure is introduced, large particles of low-quality coal (especially lignitous coal) may crack and form a large quantity of coal dust during the drying process. When the coal dust is mixed into the dense medium, not only the coal dust itself can't be separated, but also the density stability of the separation bed layer is severely degraded, thus the separation effect will get worse.